ipblocklist-functions.pl: Allow downloading empty blocklists

ipblocklist-functions.pl: Allow downloading empty blocklists

Some blocklist providers does serve blocklists for current events or
with very limited updates. Therefore there is a chance such a blocklist
could be empty for a certain time.

This patch allows to replace an existing filled blocklist by an empty
one and vice versa.

Fixes #13804.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>